As illustrated in FIG. 1, a nacelle 10 comprises a front air intake 12 making it possible to channel a flow of air toward the engine.
Following a longitudinal cross-section (containing the axis of rotation of the engine), an air intake 12 comprises a lip 14 extended outside the nacelle by an outer wall 16 and inside by the inner wall 18 defining an inner duct 20 making it possible to channel the air toward the engine.
The nacelle also comprises a front frame 22 that delimits an annular duct 24 with the lip 14 that can be used to channel hot air for frost treatment.
To limit the impact of noise annoyances, techniques have been developed to reduce internal noise, in particular by arranging panels or coverings at the walls of the inner duct 20, aiming to absorb part of the sound energy, in particular by using the principle of Helmholtz resonators.
To optimize acoustic treatment, these panels must cover the largest possible surface. Some acoustic treatment panels 26 may cover the inner duct 20, those panels remote the front frame not performing a frost treatment function. In that case, an acoustic panel comprises, from the outside toward the inside, an acoustically resistive layer 28, at least one cellular structure 30, and a reflective layer 32.
Other panels may be arranged in the annular duct at the front of the frame 22 and combine the acoustic treatment and frost treatment functions. Lastly, an acoustic treatment panel 34 with heat resistant materials may be inserted between the front frame 22 and the panels 26. That panel 34 also incorporates the frost treatment function and comprises means for capturing the hot air in the annular duct 24 and expelling it toward the rear in the inner duct 20.
Such a panel combining the acoustic treatment and frost treatment functions is in particular described in patent FR-2,917,067. It comprises, from the outside toward the inside, an acoustically resistive layer, at least one cellular structure, and a reflective layer, as well as channels each delimited by a wall inserted between the acoustically resistive layer and the cellular structure.
This solution makes it possible to limit the risks of communication between the inside of the channels and cells of the cellular structure, and therefore the risks of disruptions in the acoustic treatment.
In all cases, as illustrated in FIG. 2, the cellular structure 30 comprises ducts 36 oriented perpendicular to the reflective and acoustically resistive layers, one of the two emerging ends of which is covered by the acoustically resistive layer, and the other of which is covered by the reflective layer.
According to one embodiment, the cellular structure comprises partitions 38 oriented perpendicular to the reflective and acoustically resistive layers, defining a honeycomb.
According to another embodiment, the cellular structure comprises strips oriented perpendicular to the reflective and acoustically resistive layers that interlace as shown in document FR-2,912,780.
For the acoustic panel to perform well, it is necessary for the cells not to be able to communicate with each other so as to avoid a flow of air being created inside the acoustic treatment panel from a first point to a second, remote point capable of disrupting the flow of air channeled by the air intake and the inner duct 20.
As a result, the peripheral edges of the ends of each duct 36 of the cellular structure must be in contact with the reflective or acoustically resistive layer and substantially sealably connected with said layers.
In the case of an acoustic treatment panel made from a composite material, this connection between the peripheral edges of the ends of the ducts of the cellular structure and one of the two reflective or acoustically resistive layers is done by adhesion.
In the case of a metal acoustic treatment panel, this connection is done by welding or brazing. This is a delicate operation.
According to another issue, the acoustic treatment panels are generally made flat and are shaped to adapt to the shapes of the inner duct 20 or the air intake. During this shaping, given the stiffness of the honeycomb structure, the connections between the peripheral edges of the ends of the ducts of the cellular structure and the two reflective or acoustically resistive layers are subject to stresses that can damage them.
The risks of leaks between the cells are even more significant inasmuch as the cells are present at the connection of the cellular structure and the acoustically resistive layer, as well as the connection between the cellular structure and the acoustically resistive layer.